In order to maximize efficiency of limited radio resources, an effective transmission and reception scheme and various methods of utilization thereof have been proposed in a broadband wireless communication system. An orthogonal frequency division multiplexing (OFDM) system capable of reducing inter-symbol interference (ISI) with a low complexity is taken into consideration as one of next generation wireless communication systems. In the OFDM, a serially input data symbol is converted into N parallel data symbols, and is then transmitted by being carried on each of separated N subcarriers. The subcarriers maintain orthogonality in a frequency dimension. Each orthogonal channel experiences mutually independent frequency selective fading. As a result, complexity is decreased in a receiving end and an interval of a transmitted symbol is increased, thereby minimizing the ISI.
In a system using the OFDM as a modulation scheme, orthogonal frequency division multiple access (OFDMA) is a multiple access scheme in which multiple access is achieved by independently providing a part of available subcarrier to each user. In the OFDMA, frequency resources (i.e., subcarriers) are provided to respective users, and the respective frequency resources do not overlap with one another in general since they are independently provided to the multiple users. Consequently, the frequency resources are allocated to the respective users in a mutually exclusive manner. In an OFDMA system, frequency diversity for the multiple users can be obtained by using frequency selective scheduling, and subcarriers can be allocated variously according to a permutation rule for the subcarriers. In addition, a spatial multiplexing scheme using multiple antennas can be used to increase efficiency of a spatial domain.
A multiple input multiple output (MIMO) technique uses multiple transmit antennas and multiple receive antennas to improve data transmission/reception efficiency. Exemplary methods for implementing diversity in a MIMO system include space frequency block code (SFBC), space time block code (STBC), cyclic delay diversity (CDD), frequency switched transmit diversity (FSTD), time switched transmit diversity (TSTD), precoding vector switching (PVS), spatial multiplexing (SM), etc. A MIMO channel matrix depending on the number of receive antennas and the number of transmit antennas can be decomposed into a plurality of independent channels. Each independent channel is referred to as a layer or a stream. The number of layers is referred to as a rank.
As disclosed in the section 6 of 3GPP (3rd generation partnership project) TS 36.211 V8.8.0 (2009-09) “Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation (Release 8)”, examples of downlink control channels used in 3GPP LTE include a physical control format indicator channel (PCFICH), a physical downlink control channel (PDCCH), a physical hybrid-ARQ indicator channel (PHICH), etc. The PCFICH transmitted in a first OFDM symbol of a subframe carries information regarding the number of OFDM symbols (i.e., a size of a control region) used for transmission of control channels in the subframe. Control information transmitted through the PDCCH is called downlink control information (DCI). The DCI indicates uplink or downlink scheduling information, an uplink transmit power control command for any UE groups, etc. The PHICH carries an acknowledgement (ACK)/non-acknowledgement (NACK) signal for uplink hybrid automatic repeat request (HARQ). That is, the ACK/NACK signal for uplink data transmitted by a user equipment (UE) is transmitted through the PHICH.
A plurality of PHICHs can be transmitted according to a system environment. In particular, there is a need to transmit the plurality of PHICHs simultaneously in a carrier aggregation system for transmitting data by using a plurality of carriers, a MIMO system, etc. A base station (BS) allocates resources to the plurality of PHICHs, and transmits ACK/NACK through the PHICH.
When considering a cross-scheduling scheme in the carrier aggregation system, there may be a problem in that a plurality of PHICHs are allocated to the same resource when using the conventional method of transmitting ACK/NACK through the PHICH. Accordingly, there is a need to consider an ACK/NACK transmission method capable of solving this problem.